a good indicator of what their actual nutritional condition is 
in the river. And here, nitrogen was deficient. The control 
tank to which we added nothing whatsoever gave no response, nor 
did the P-tank. So we can add phosphorus in the summer period 
and not get a response whatsoever. 
In wintertime we see a weak response to phosphorus rather 
late in an experiment. For nitrogen, there is no response what¬ 
soever . 
This is completely in keeping with the availability of nitro¬ 
gen or phosphorus that I showed above. In the spring and fall, 
we find that neither N or P stimulates growth. Neither phospho¬ 
rus nor nitrogen addition is very important, as one would also 
expect from monitoring data. 
I can summarize for you what we have seen in terms of nu¬ 
trient limitation over the annual cycle in the Patuxent River. 
In Figure 9, the ordinal scale represents a relative indication 
of nutrient limitation in the Patuxent. The higher the index 
value, the more likely that nutrient is limiting by our enrich¬ 
ment studies. The first summer we saw a strong response to 
nitrogen addition and very little to phosphorus. 
Late fall saw no response to nutrient enrichment. In the 
winter season of 1983-1984, we saw some response to phosphorus, 
but on a relative level, that response was less than nitrogen 
the previous summer. In the springtime, there was no response 
to the addition of either nutrient. The following year, we got 
a large response to nitrogen and very, very little response to 
phosphorus. 
Figure 10 clearly shows that we get a much greater response 
to addition of nitrogen to our system than we do to phosphorus. 
The system, therefore, is likely to be a nitrogen-limited sys¬ 
tem. 
What does this mean in terms of its management implications? 
Well, I think there is a very simple take-home lesson there. If we 
are really going to try to control the anoxia problem, at least in 
the tributaries, we must control the growth of algae. To control 
the growth of algae, we have to remove what's available in least 
supply — the "limiting nutrient". That is nitrogen, I think, for 
most of the saline regions of the Patuxent River and probably for 
elsewhere in the saline regions of the Chesapeake Bay during the 
low-flow season. 
So we're going to need to develop some strategy to control N 
inputs. I have not included in the program and discussions of how 
one might go about doing this. Generally speaking, N removal has 
been regarded as a very, very expensive and difficult process to do 
both for point and non-point sources. I think there is some 
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